A process for producing unsaturated aldehydes and/or unsaturated acids from olefins or alkanes in vapor phase by using a catalyst is a typical process of catalytic vapor phase oxidation.
Particular examples of such catalytic vapor phase oxidation include a process for producing acrolein and/or acrylic acid by the oxidation of propylene or propane, or a process for producing methacrolein and/or methacrylic acid by the oxidation of isobutylene, isobutane, t-butyl alcohol or methyl t-butyl ether.
Generally, catalytic vapor phase oxidation is carried out by charging one or more kinds of granular catalysts into a reaction tube (catalytic tube), supplying feed gas into a reactor through a pipe, and contacting the feed gas with the catalyst in the reaction tube. Reaction heat generated during the reaction is removed by heat exchange with a heat transfer medium whose temperature is maintained at a predetermined temperature. The heat transfer medium for heat exchange is provided on the outer surface of the reaction tube so as to perform heat transfer. The reaction mixture containing a desired product is collected and recovered through a pipe, and sent to a purification step. Since the catalytic vapor phase oxidation is a highly exothermic reaction, it is very important to maintain reaction temperature within a certain range and to reduce the magnitude of a hot spot occurring in a reaction zone. It is also very important to disperse heat at a site where heat accumulation may occur due to the structure of a reactor or a catalyst layer.
The partial oxidation of olefins or alkanes corresponding thereto uses a multimetal oxide containing molybdenum and bismuth or vanadium or a mixture thereof, as a catalyst.
Generally, (meth)acrylic acid, a final product, is produced from propylene, propane, isobutylene, isobutane, t-butyl alcohol or methyl-t-butyl ether (referred to as ‘propylene or the like’, hereinafter) by a two-step process of vapor phase catalytic partial oxidation. More particularly, in the first step, propylene or the like is oxidized by oxygen, inert gas for dilution, water steam and a certain amount of a catalyst, so as to produce (meth)acrolein as a main product. Then, in the second step, the (meth)acrolein is oxidized by oxygen, inert gas for dilution, water steam and a certain amount of a catalyst, so as to produce (meth)acrylic acid. The catalyst used in the first step is a Mo-Bi-based oxidation catalyst, which oxidizes propylene or the like to produce (meth)acrolein as a main product. Also, some acrolein is continuously oxidized on the same catalyst to partially produce (meth)acrylic acid. The catalyst used in the second step is a Mo-V-based oxidation catalyst, which mainly oxidizes (meth)acrolein in the mixed gas containing the (meth)acrolein produced from the first step to produce (meth)acrylic acid as a main product.
A reactor for performing the aforementioned process is provided either in such a manner that both the two-steps can be performed in one system, or in such a manner that the two steps can be performed in different systems.
Recently, a catalyst for use in producing unsaturated acids such as (meth)acrylic acid from alkanes such as propane or isobutane via a single-step process has been developed.
Meanwhile, (meth)acrylic acid manufacturers now conduct diversified efforts either to improve the structure of the reactor so as to increase the production of acrylic acid by the reactor, or to propose the most suitable catalyst to induce oxidation, or to improve process operations.
In part of such prior efforts, propylene or the like which is supplied into the reactor is used at high space velocity and high concentration. However, in this case, rapid oxidation occurs in the reactor, which makes it difficult to control the resultant reaction temperature. Also, a hot spot is generated in the catalyst layer of the reactor, and heat accumulation occurs in the vicinity of the hot spot, resulting in increased production of byproducts, such as carbon monoxide, carbon dioxide and acetic acid at high temperature, and in a drop in yield of (meth)acrylic acid.
Furthermore, production of (meth)acrylic acid using high space velocity and high concentration of propylene or the like causes various problems, as the reaction temperature abnormally increases in the reactor, such problems including the loss of active ingredients from the catalyst layer, a drop in the number of active sites caused by sintering of metal components, or the like. Consequently, this leads to deterioration of the function of the catalyst layer.
Accordingly, in the production of (meth)acrylic acid, control of the reaction heat in the relevant reactor is of great importance. Particularly, not only the formation of hot spots in the catalyst layer but also the accumulation of heat in the vicinity of the hot spots must be inhibited, and the reactor must be effectively controlled so that the hot spots do not lead to reactor runaway (a state where the reactor cannot be controlled or explodes by a highly exothermic reaction). Therefore, it is very important to inhibit hot spots and heat accumulation in the vicinity of the hot spots so as to extend the lifetime of the catalyst, to inhibit side reactions, and thus to increase yield of (meth)acrylic acid.